1. Field of the Invention
The present invention relates to a control device for an internal combustion engine which is provided with a VVT (variable valve timing) mechanism, and more specifically, to an estimation device for a cylinder intake air amount in an internal combustion engine, which serves for calculating an amount of intake air in a cylinder with a high degree of accuracy.
2. Description of the Related Art
In general, in order to control an engine in a suitable manner, it is important to calculate an amount of air to be sucked into a cylinder with a high degree of accuracy, and to carry out fuel control and ignition timing control according to the amount of air which has been sucked into the cylinder.
With respect to the fuel control, if fuel can be controlled in a feedback manner so that an amount of fuel is injected to provide a target air fuel ratio with respect to the amount of intake air sucked into the cylinder (hereinafter also referred to as a cylinder intake air amount), generally good controllability will be obtained.
On the other hand, with respect to the ignition timing control, there is a need to control ignition timing to an advance angle (MBT: Minimum Advance for Best Torque) so as to provide a maximum output power of the engine according to not only an engine rotational speed and an amount of intake air sucked into a cylinder, but also other factors including, for example, the temperature of the engine, the situation of the occurrence of a knock, the property of fuel, and the rate of EGR (Exhaust Gas Recirculation) which is the ratio of an amount of EGR with respect to the amount of intake air).
Among the above-mentioned factors which have an influence on the MBT, for example, the temperature of the engine can be detected by an engine cooling water temperature sensor, and the situation of the occurrence of a knock can be detected by a knock sensor, and the property of fuel can be determined as regular gasoline or high-octane gasoline according to the situation of the occurrence of a knock.
However, with respect to the rate of EGR, an external EGR can be calculated from the degree of opening of an EGR valve, exhaust gas pressure, and the internal pressure of an intake pipe, but it is difficult to estimate an internal EGR rate with a high degree of accuracy.
That is, with respect to the amount of EGR, there are external EGR control which is based on the degree of opening of the EGR valve arranged in an EGR passage connecting an exhaust pipe and the intake pipe with each other, and internal EGR control which serves to control the amount of exhaust gas remaining in the cylinder. In addition, the external EGR control and the internal EGR control may be carried out at the same time.
The control of the internal EGR is carried out as follows. That is, a VVT (Variable Valve Timing) mechanism is provided which serves to make variable the valve opening and closing timing of an intake valve and an exhaust valve, so that the internal EGR is controlled by changing an overlap period in which the intake valve and the exhaust valve are in their open states at the same time according to the valve opening and closing timing.
In particular, in an engine having an intake and exhaust VVT system which serves to make variable the valve opening and closing timing of an intake valve and an exhaust valve, an amount of air sucked into a cylinder from an intake manifold changes to a large extent depending on the valve opening and closing timing of the intake and exhaust valves, as a result of which in cases where no consideration is given to the influence due to the valve opening and closing timing, the accuracy in the calculation of the amount of air sucked into the cylinder may decrease.
In addition, in recent years, it becomes general to carry out engine control by using engine output torque as an index, and in cases where the engine output torque is estimated, too, thermal efficiency changes according to the cylinder intake air amount and the EGR rate, and hence, in order to calculate the above-mentioned MBT as well as to estimate the torque and the thermal efficiency, too, it is required that the cylinder intake air amount and the EGR rate should be calculated with a high degree of accuracy.
In the past, there has been proposed an estimation device for a cylinder intake air amount in an internal combustion engine in the case where the estimation device is intended to be used for an engine which has an intake and exhaust VVT system (for example, refer to a first patent document and a second patent document to be described later).
In the first patent document, there is disclosed a technique in which a physical model, which models a response delay of an intake system until the air having passed through a throttle valve comes into a cylinder, is used as an arithmetic operation (calculation) model, and the arithmetic operation model is calibrated based on the pressure within an intake air path.
According to the above-mentioned first patent document, it is considered that an error in a volumetric efficiency correction factor can be absorbed, but in the case of the intake and exhaust VVT system, when taking as an example the case where the number of divisions for control in each of the intake air and the exhaust gas is six (in cases where the operating range of a VVT mechanism is represented by six representative points and is interpolated between these adjacent points), data for 6×6 (=36) pieces of volumetric efficiency correction factor maps are required as basic characteristics.
Here, note that six volumetric efficiency correction factor maps are only required for a system configuration having only an intake VVT mechanism.
On the other hand, in the second patent document, there is disclosed a technique in which, unlike the first patent document, by the use of an air flow meter which serves to detect an amount of intake air in an engine, and an intake air pressure sensor which serves to detect a pressure of the intake air in the engine, the amount of intake air thus detected is corrected by an amount of change of the intake air pressure, and the result of the correction operation is integrated in the course of intake stroke, whereby a cylinder intake air amount is calculated for each of cylinders.
According to the above-mentioned second patent document, it appears that the calculation of a volumetric efficiency correction factor and the filtering processing of a response delay model are unnecessary, but with sensors and the like used in engine control, there are many cases in which fine or minute errors may often occur at the time of measurements, and hence, if appropriate countermeasures are taken, it is difficult to calculate the cylinder intake air amount with a high degree of accuracy.
For example, in particular for sensor values of an intake system for which high accuracy is required, processing is carried out for suppressing the fine or minute errors thereof, such as, for example, sampling them every 1 ms and calculating an average value of sampled values between every predetermined crank angles. In addition, it can be said that this has the same effect with respect to the filtering processing of the response delay model.
That is, it is considered that with the technique of the second patent document, a fine or minute error occurs in the cylinder intake air amount, and it is not suitable as it is for fuel control and ignition timing control, and moreover, it is not suitable for use in the estimation arithmetic operation of an engine output torque.